During tobacco ripening or curing the expression of various genes is altered. Such genes may affect metabolic pathways involved in the formation of numerous secondary metabolites including terpenoids, polyphenols, and alkaloids that affect end-product quality traits. For example, the bioconversion of nicotine to form nornicotine during plant senescence and in the post-harvest or leaf curing phase occurs in many Nicotiana species. Nicotine is the predominant source of nornicotine. The nornicotine alkaloid, is a substrate for microbe-mediated nitrosation to form the tobacco specific nitrosamine (TSNA) N′-nitrosonornicotine (NNN) during leaf curing or subsequent leaf storage and processing.
Genes expressed during tobacco ripening or curing may be constitutively expressed, ethylene-induced or senescence-related genes, for instance, genes encoding a cytochrome p450. Cytochrome p450s, for example, catalyze enzymatic reactions for a diverse range of chemically dissimilar substrates that include the oxidative, peroxidative, and reductive metabolism of endogenous and xenobiotic substrates. In plants, p450s participate in biochemical pathways that include the synthesis of plant products such as phenylpropanoids, alkaloids, terpenoids, lipids, cyanogenic glycosides, and glucosinolates studied (Chappell, Annu Rev. Plant Physiol. Plant Mol. Biol. 46:521-547, 1995). Cytochrome p450s, also known as p450 heme-thiolate proteins, usually act as terminal oxidases in multi-component electron transfer chains, called p450-containing monooxygenase systems. Specific reactions catalyzed by these enzyme systems include demethylation, hydroxylation, epoxidation, N-oxidation, sulfooxidation, N-, S-, and O-dealkylations, desulfation, deamination, and reduction of azo, nitro, and N-oxide groups.
The diverse role of Nicotiana plant p450 enzymes has been implicated in effecting a variety of plant metabolites such as phenylpropanoids, alkaloids, terpenoids, lipids, cyanogenic glycosides, glucosinolates, and a host of other chemical entities. Some p450 enzymes can impact the composition of plant metabolites. For example, it has been long desired to improve the flavor and aroma of certain plants by altering a plant's profile of selected fatty acids through breeding; however very little is known about mechanisms involved in controlling the levels of these leaf constituents. The down regulation or up regulation of p450 enzymes associated with the modification of fatty acids may facilitate accumulation of desired fatty acids that provide more preferred leaf phenotypic qualities.
The function of p450 enzymes and their broadening roles in plant constituents is still being discovered. For instance, a special class of p450 enzymes was found to catalyze the breakdown of fatty acid into volatile C6- and C9-aldehydes and β-alcohols that are major contributors of “fresh green” odor of fruits and vegetables. The level of other novel targeted p450s may be altered to enhance the qualities of leaf constituents by modifying lipid composition and related breakdown metabolites in Nicotiana leaf. Several of these constituents in leaf are affected by senescence that stimulates the maturation of leaf quality properties. Still other reports have shown that p450s enzymes are playa functional role in altering fatty acids that are involved in plant-pathogen interactions and disease resistance.
The large multiplicity of p450 enzyme forms, their differing structure and function have made their research on Nicotiana p450 enzymes very difficult before the present invention. In addition, cloning of p450 enzymes has been hampered at least in part because these membrane-localized proteins are typically present in low abundance and often unstable during purification. Hence, a need exists for the identification of p450 enzymes in plants and the nucleic acid sequences associated with those p450 enzymes. In particular, only a few cytochrome p450 proteins have been reported in Nicotiana. The inventions described herein entail the discovery of cytochrome 450s and cytochrome p450 fragments that correspond to several groups of p450 species based on their sequence identity.
In addition to the p450 sequences, the present invention encompasses the discovery of other constitutive and ethylene or senescence induced sequences that address the need for regulating metabolic pathways involved in the formation of secondary metabolites that affect the quality of a tobacco product. These sequences are also useful in the development of plant germplasms that have desirable traits for use in breeding programs to develop more desirable germplasms, and especially non-GMO (genetically modified organism) type germplasms.